I think that on the one hand Frank did not want to go to see the play because the actors working in it were amateurs . However, it sounds like an excuse for me for I believe Frank might have started feeling attracted towards Rita and he really enjoys her company, therefore going out together might cause him to fell more involved than in a teacher- student relationship.
From the educational point of view, this reflects the prejudices we might have against certain student. At times teachers just label students without seeing beyond their school life causing them frustration and anger, making their own job even more difficult. Although it is not an easy taks, we should be able to accompany each student individually, see their needs and different capacities and then teach and evaluate accordingly...